Mount Prospect, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

179
FXUS63 KLOT 171800 CCA
AFDLOT

Area Forecast Discussion...CORRECTED
National Weather Service Chicago/Romeoville, IL
100 PM CDT Wed Jun 17 2026

.KEY MESSAGES...

- An outbreak of severe thunderstorms with the potential to
  produce damaging tornadoes, destructive straight line winds,
  and large hail is possible this afternoon into the early
  evening.

- A flood watch has been issued for today. Locally heavy
  rainfall is expected to result in an enhanced threat of
  localized flash flooding.

- Tranquil weather is expected to close out the work week in the
  wake of this system.

- Another strong storm system is expected to bring a return of
  showers and thunderstorms later this weekend, with some
  potential for heavy rainfall and/or severe weather somewhere
  in the general region.

&&

.UPDATE...
Issued at 1100 AM CDT Wed Jun 17 2026

Now pushing through the latter half of the morning, we find a
mature bowing MCS diving southeastward across central IL. This
line has been producing torrential rainfall and damaging wind
gusts since maturing over central Iowa during the predawn hours.
The system is riding along the northern flank of the MUCAPE
gradient with little to no elevated instability coincident with
the core of the complex, at least until just recently. The 12Z
RAOB out of DVN sampled no instability through the column, but
it certainly picked up on the impressive shear profile working
into the region. With the limited instability, this system is
being largely driven by strong kinematic forcing, very efficient
moist upglide, and a likely gravity wave influence. Latest SPC
mesoanalysis depicts a 60kt SW 850mb jet nosing into western IL
beneath 70+ kt of diffluent WNW upper level flow downstream of
the approaching jet max. These features make for strong vertical
lift through the column. This low-mid level flow is resulting
in impressive moisture/theta-e advection feeding into this
complex with 100mb mean mixing ratios near 17g/kg and PWATs of
1.7" and climbing. The resultant heavy downpours with MRMS
picking up on 3 to 4" per hour rates recently prompted a number
of flash flood warnings to be issued across parts of SE IA and
western IL.

The severe bowing segment will continue to across central IL
through the end of the morning along the instability gradient,
which includes southern and southwestern portions of our CWA.
MLCAPE building north across central IL with time coupled with
elongating hodographs could result in a tornado threat along the
leading edge of the line in the coming hours. A Tornado Watch
was recently issued around our southwestern CWA for this
potential. However, most recent guidance suggests the MLCAPE
gradient should remain safely south of our CWA through the
morning and damaging wind gusts will continue to be our primary
concern with this current push. The flash flood threat will also
translate into our area with this line and favor areas south of
I-80 through the morning.

Additionally, a wake low appears to be moving into northern IL
behind the showers, north of the severe bowing segment.
Widespread 2- hour surface pressure falls of 3 to 5 mb have
been observed across northern and eastern IA with a couple as
high as 7 mb. Additionally, a number of sites and personal
weather stations have been recording 50+ mph gusts out of the
southeast in the wake. Accordingly, we issued a Wind Advisory
through 1 PM for areas along and north of I-88 for non-
thunderstorm wind gusts up to 50 mph. As things are shaping up,
these wake low winds may be our overall biggest concern through
the rest of the morning.

Beyond the morning hours, the going forecast remains on track,
although many of the same uncertainties still linger. The big
question of the day is how well the atmosphere will recover into
northern IL following this morning convection. Most of the
newest CAM guidance from this morning has the effective warm
front, and by extension MLCAPE gradient, locking up well south
of I-80, if not just south of our CWA boundary this afternoon.
There is still enough support though for the front to surge
farther north and open up more of the CWA to the more favorable
severe environment. The RAP has consistently been farther north
in the position of the front this afternoon parking it near the
I-88 corridor. While an outlier among CAM guidance, it`s hard to
argue this possibility given the kinematic field at play. The
front is currently sharpening up over south-central Illinois,
but we will unfortunately have to wait until much of this
current activity clears away before we can hope to gain a handle
on trends into later today with the augmented front, but steady
rain and storms will continue to muddy up our insight and make
it extraordinarily difficult to anticipate trends. Still
anticipate a highly favorable severe environment this afternoon
near and south of the warm front. As additional storms develop
along and ahead of the impending cold front, those in the warm
sector will be exposed to uncapped instability and outlandishly
long, looping hodographs capable of resulting in destructive
wind gusts and strong, long tracked tornadoes. The flash flood
potential remains alive and well through the day across the CWA
regardless of the position of the front. See the full discussion
below for finer details regarding severe threats through the
afternoon. Regular updates will be made to the forecast to
reflect latest thinking.

Doom

&&

.DISCUSSION...
Issued at 212 AM CDT Wed Jun 17 2026

Through this evening:

Complex set-up today with difficult to predict mesoscale
details regarding early day convection and its impacts on
surrounding environment likely to play a key role in the
magnitude and location of severe potential this afternoon and
evening. Significant regional severe weather outbreaks are
often a result of mesoscale "accidents" happening withing an
otherwise favorable synoptic scale environment for severe
weather.

Synoptically, today fits that bill well with an unseasonably
strong 120kt+ 250mb upper level jet nosing east-southeast into
the mid-Mississippi Valley this afternoon. A rather sharp and
vigorous 500mb trough will reach the Upper Mississippi Valley
later this afternoon, with a remarkably strong (for mid June)
80kt+ H5 jet streak rounding the base of this trough. At the
sfc, a near record deep (for June) double barreled low pressure
system is progged by 00z to have one low over southern Lake
Michigan east of the IL/WI border with a trough extending
northwest to a second low over northwest Wisconsin. From a large
scale perspective, this set-up looks exceptionally favorable
for a regional severe weather outbreak including the potential
for intense tornadoes, however the devil may end up being in the
mesoscale details that unfortunately just cannot be accurately
predicted this far in advance.

A strong low level jet over the eastern Plains is rapidly
transporting deep moisture northward into the lower Missouri
Valley. This process can be seen well on GOES TPW imagery. The
combination of strong large scale ascent associated with the
upper level jet and strong isentropic ascent with the
aforementioned low level jet has already led to the development
of showers and scattered thunderstorm from northern IA west-
northwest across South Dakota and southern Minnesota. Convective
allowing models have a pretty strong signal depicting more
vigorous, potentially severe, convection developing over eastern
IA just prior to dawn this morning as this strong forcing
encounters a rapidly northeastward expanding axis of elevated
instability driven by the strong low level theta-e advection
from the low level jet. The severe threat with this convection
looks like it will be confined more to areas south and west of
our CWA, closer to the expected elevated instability axis, but
cannot rule out a severe threat later this morning creeping
into our far southwestern CWA.

There is considerable spread in guidance in how this convection
evolves, but with low level jet remaining strong through the day
as the nose shifts east into Illinois, it is certainly plausible
that this convection will increase in coverage and organize into
an MCS. Depending on the track, longevity, and intensity of the
potential MCS, the synoptic scale environment will be disrupted
heading into the afternoon. Again, there`s considerable spread
in guidance with respect to the placement of what will likely
become the effective boundary, a composite of the synoptic scale
warm front and outflow boundary from this MCS. A number of
models indicate an extremely rapid northward surge of low level
moisture and result instability in the wake of this MCS, while
other guidance indicates that the effective boundary may
struggle to even reach the Highway 24 corridor across our far
southern CWA.

Normally it is safe to expect a farther south than progged
location of the effective boundary in the wake of an early
afternoon MCS, however today`s set-up features an unseasonably
deep surface low for mid June with a near peak high sun angle.
These factor make it difficult to write off the models that
depict this boundary and unstable air mass surging rapidly north
in the wake of the MCS, possibly making it to nearly the
Wisconsin border later this afternoon. The northward extent of
the boundary should be demarcation between a more substantial
severe weather and tornado threat, and a potentially more muted
(but still existent) severe weather threat to the north.

Several of the CAMs are depicting convective initiation along a
pre-cold front trough in the warm sector. Given the expected
shear, should air mass recovery actually be sufficient, then
this would create an exceptionally favorable environment for
discrete, fast moving tornadic supercells. Given how quickly
these cells will tend to propagate off of the initiating
boundary, their ability to survive and thrive will hinge on them
traversing a sufficiently unstable/weakly capped environment.
Where and whether this type of environment will exist, will
hinge on how the difficult to predict mesoscale details play out
this afternoon. Also, the magnitude of instability will play a
role in how quickly these storm develop along the boundary as
low level convergence along the prefrontal trough isn`t progged
to be terribly strong. So if weak convection quickly moves off
the boundary to even a weakly capped air mass, it could struggle
to mature. Where and whether this type of environment will
exist, will hinge on how the difficult to predict mesoscale
details play out this afternoon.

Forecast hodographs within the warm sector feature an amazingly
strong low level shear profile with 0-1km SRH well over 300
m^2/s^2, maximized closest to the composite warm front/outflow
boundary where some backing of the low level flow is likely.
Given that LCLs and LFCs will be quite low, there could be
rather strong low level CAPE despite the progged abysmal low
level lapse rates (less than 6C/km).

Ultimately, today is going to require close monitoring of the
mesoscale environment to determine if and where the greatest
tornado threat will exist this afternoon and early evening. The
current SWODY1 placement of the MDT risk highlights the most
likely area within our CWA to face this potential, however it
also wouldn`t be hard to envision a scenario where early
convection results in the more favorable environment developing
farther south/east than the current MDT risk. Given these
uncertainties, not planning any significant change to our
messaging for the severe potential at this time, however as
observational trends begin to shed a light on the ultimate
evolution of the mesoscale environment, some changes to today`s
severe whether forecast may be needed.

Finally, the influx of richer Gulf moisture combined with the
strong synoptic forcing continues to look favorable for locally
heavy rainfall totals today. While storm motion will be fast,
rainfall efficiency will be high, so localized totals in excess
of 3" (most of it falling in a very short time) are certainly
well within the realm of possibility. Flash flood guidance
remains low, so in coordination with neighboring offices will be
issuing a Flood Watch for today.

Beyond this evening:

Gusty westerly winds are expected overnight as the deep low
pulls away. Dry and pleasant weather is expected Thursday and
Friday. Another strong shortwave trough is progged to move
across the Midwest later this weekend, likely resulting in
another round of potentially heavy rainfall and/or severe
weather in the general region. Too soon to say how much, if any
of our CWA will be impacted, but our area is certainly well
within the proverbial cone of uncertainty and pops are very high
Sunday accordingly.

- Izzi

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1244 PM CDT Wed Jun 17 2026

Key Messages:

- Strong SE winds lingering through about 19z near MDW/GYY

- Showers and embedded thunderstorms expanding in coverage
  across the Chicago-area terminals this afternoon.

- SW wind shift this afternoon and then a NW wind shift
  overnight into Thursday morning.

- Chances for showers/thunderstorms this evening.

- Periods of MVFR cigs through Thursday morning.

A lingering burst of strong southeasterly winds near MDW/GYY
will gradually subside through 19z. The threat for embedded
thunderstorms will then increase after 19-20z. The storm threat
should then shift south of the c90 TRACON by 23-00z. Winds will
then shift out of the southwest and ease late this afternoon.

Later this evening, winds will increase once again and MVFR cigs
are expected to re-develop across the region late tonight into
Thursday morning. There is a small chance for showers and a few
thunderstorms  this evening as a front moves across the region,
but chances remain a bit too low for TEMPO groups. Rough timing
for this would be about 23-01z at RFD and 01-03z at the Chicago-
area sites.

MVFR cigs should lift to VFR through late Thursday morning.

Carlaw

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch until 5 PM CDT this afternoon for ILZ003-ILZ004-
     ILZ005-ILZ008-ILZ010-ILZ011.

     Wind Advisory until 1 PM CDT this afternoon for ILZ003-ILZ004-
     ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ103-
     ILZ104.

     Flood Watch until 9 PM CDT this evening for ILZ006-ILZ012-
     ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-
     ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

     Beach Hazards Statement through late tonight for ILZ006-ILZ103-
     ILZ104.

IN...Flood Watch until 9 PM CDT /10 PM EDT/ this evening for INZ001-
     INZ002-INZ010-INZ011-INZ019.

     Beach Hazards Statement from late tonight through Thursday
     afternoon for INZ002.

LM...Small Craft Advisory until 9 AM CDT Thursday for the IL and IN
     nearshore waters.

&&

$$

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